1. Field of the Invention
The present invention relates to an amplifier for FM antennas.
2. Description of the Related Art
Any amplifier for FM antennas is directly connected to an FM antenna. The amplifier amplifies any signal that the antenna has receives. The signal amplified is supplied to the tuner circuit connected to the output of the amplifier. FIG. 6 is a circuit diagram illustrating an amplifier for FM antennas.
The circuit shown in FIG. 6 is one for use in automobiles. In the circuit, a signal that an FM antenna (not shown) receives is supplied via a capacitor C1 to the base of an NPN transistor Tr1.
The transistor Tr1 has its emitter connected to the ground by a resistor R1. A resistor R2 connects the collector and base of the transistor Tr1. An operating voltage +12[V] is applied from a DC power supply PS to the collector of the transistor Tr1 through a resistor R3. The resistor R3 and the DC power supply PS are connected to one end of a capacitor C2. The other end of the capacitor C2 is connected to the ground.
A resistor R4 and a capacitor C3 connect the emitter of the transistor Tr1 to the output terminal. The output terminal is connected to a tuner circuit (not shown).
The amplifier for FM antennas is designed to exhibit improved strong-input characteristic. It excels in strong-input characteristic such as FM secondary intermodulation distortion characteristic (FN-IM2) or FM tertiary intermodulation distortion characteristic (FM-IM3).
The amplifier is, however, greatly inferior in signal-amplifying characteristic, noise-figure characteristic and the like. It is not satisfactory in terms of all characteristics.
An object of this invention is to provide an amplifier for FM antennas, which has no complex circuit configuration, which yet excels in not only strong-input characteristic but also signal-amplifying characteristic and noise-figure characteristic, and whose characteristics are well balanced.
An amplifier for FM antennas, according to this invention, comprises a transformer and a plurality of field-effect transistors. An input terminal is connected to one end of the primary winding of the transformer. A power-supply voltage is applied to one end of the secondary winding of the transformer. A given middle part of the secondary winding is connected to an output terminal. The transistors have their source connected in parallel to the other end of the primary winding of the transformer, their drains connected in parallel to the other end of the secondary winding, and their gates connected to the ground.